1. Field of the Invention
Monochromators are mainly used in photoelectric spectrometers, sometimes called spectrophotometers. A monochromator serves to disperse polychromatic light for the purpose of selecting light of more or less uniform wavelength -- monochromatic light -- and transmitting this light further in the apparatus for measuring purposes.
The actual spectral dispersion in the monochromator is done either by a prism or be a diffraction grating. When reference is made hereinafter to a "grating", it is to be understood that fundamentally a prism can be used instead.
2. Description of the Prior Art
A conventional monochromator consists of a light entry aperture, the grating, the exit aperture and lenses and/or mirrors for focusing and directing the light beams.
In known monochromators the wavelength selection of the monochromatic light is effected by turning the dispersing element with respect to the incident light beam in such a manner that the light of desired wavelength is incident exactly on the exit aperture. Light of all other wavelengths is absorbed by the walls of the monochromator, i.e., is lost.
For certain purposes photoelectric spectrometers require light of two different monochromatic wavelengths. An example of this is a chromatogram spectrometer in which the chromatogram is scanned with light of two different wavelengths. The one wavelength is chosen so that it lies in the region of the absorption maximum of the substance to be investigated and the other so that it detects the disturbing influences of the chromatogram matrix. The signals of the two wavelengths are correlated by means of electronic processors so that an optimum signal/noise ratio and thus a high measuring accuracy is achieved.
In known "two-wavelength apparatuses" of this type two monochromators were used for producing the two monochromatic radiations. For further considerations it is of no consequence whether for this purpose light from two radiation sources is used, or light from only one source divided by a mirror type beam splitter or equivalent means, which is the usual case, or whether the beam splitting is effected prior or after passage through the medium to be investigated. In each case, two monochromators are required in these known apparatuses.
U.S. Pat. No. 3,880,523 discloses an apparatus referred to as a colorimeter in which a light dispersing diffraction grating lies opposite a rectilinear screen receiving the spectrum. This screen has a number of larger and smaller circular openings to which light conducting elements leading to a detector head can be selectively connected. Since the openings must have a certain spacing with respect to each other it is not possible to pick up spectral wavelength ranges incident on the intermediate spaces. In addition, the light yield is not uniform because of non-uniform angles of incidence of the light beam. The band width also changes depending on the angle of incidence of the beam on the holes. All these factors prevent uniform or easily comparable measuring results.
In a second embodiment of this known apparatus a single light guide element is movable transversely across the straight screen receiving the spectrum. In this case as well, uniform measuring results cannot be obtained because the beam impinges on the entry gap of this single light guide element at different angles for various positions.